The present invention relates to a CDMA receiving apparatus, which is used in a radio communication system, and relates to a CDMA receiving apparatus.
In recent years, a radio communication system such as a cellular phone and a car phone and the like has become rapidly widespread. In the radio communication system, code division multiple access (CDMA), which has received attention recently, is a system in which a transmitting side radio transmits a wide band signal, which is secondary modulated by a spreading code, and a receiving side obtains a narrow band signal by multiplying the received signal by the same spreading code as that of the transmitting side.
In the CDMA radio communication system, a base station always transmits a control signal for establishing synchronization subjected to secondary spreading by use of the known spreading code. FIG. 1 shows a frame configuration view of the control signal for establishing synchronization transmitted from the base station in CDMA system.
As shown in FIG. 1, in the control signal, a scrambling code and a spreading code are super-imposed on data, and a first search code (FSC) for establishing slot/chip synchronization and a second search code (SSC) for identifying the scrambling code and establishing frame synchronization are multiplexed in a predesignated part of symbol such as a top portion of a slot.
In this case, the scrambling code is a long cycle spreading code, which is longer than the frame cycle and which is also referred to as long code. Similarly, the spreading code is a short cycle spreading code, which is equal to one symbol time, and which is also referred to as short code.
In the radio communication system, a mobile station establishes synchronization with a base station to which the mobile station belongs when power is on (hereinafter referred to as xe2x80x9cinitial synchronous modexe2x80x9d), and establishes synchronization with a base station of a destination at the time of movement between cells (hereinafter referred to as xe2x80x9cstandby modexe2x80x9d).
FIG. 2 is a view showing the conception of initial synchronization mode. In FIG. 2, base stations 1, 2 and 3 are base stations of cells A, B, C, respectively, and they always transmit a control signal for establishing synchronization subjected to secondary spreading by use of a different spreading code, respectively. Then, it is assumed that this shows the time when a mobile station 4, which is in the cell A, turns on the power.
The mobile station 4 must first establish synchronization with base station 1 to which the mobile station belongs as an initial synchronous mode.
The initial synchronous mode is under environment such that frequency offset compensation (hereinafter referred to as xe2x80x9cAFCxe2x80x9d) does not correctly operate and that there is no information of the base station to be connected. For this reason, it is important how slot/chip synchronization and frame synchronization should be established at high speed by identifying a scrambling code of the base station to be connected from all candidates for a scrambling code. In the initial synchronous mode, since the frequency with which power is turned on is low, there is no request of control on current consumption.
FIG. 3 is a view showing the conception of standby mode. In FIG. 3, base stations 5, 6 and 7 are base stations of cells D, E, F. Then, it is assumed that this shows the time when a mobile station 8, which is in communication with the base station 5, moves to cell E from cell D. The mobile station 8 must establish synchronization with the base station 6 of a destination, which is in cell E, as a standby mode.
In the standby mode, it is important how current consumption should be controlled in view of extension of standby time and that of communication time. The standby mode is under environment such that AFC correctly operates and that there is information of scrambling codes of adjacent base stations, and that slot/chip synchronization and frame synchronization are known to some extent. For this reason, there is no request of establishing high-speed synchronization.
The conventional CDMA receiving apparatus uses a matched filter that can identify the scramble code at high speed so as to establish initial synchronization at high speed.
However, the matched filter has a high-speed synchronous characteristic but provides a defect in which current consumption is high. Then, the conventional CDMA receiving apparatus also uses the matched filter to identify the scramble code at the standby time, causing a problem in which current consumption is increased.
It is an object of the present invention is to provide a CDMA receiving apparatus and its CDMA receiving method that is capable of improving a reduction in current consumption.
The above object can be achieved by performing the switching of two kinds of correlators adaptably according to a mode of a mobile station in order that a matched filter, which has a high-speed synchronous lead characteristic, is used at an initial synchronous time and a plurality of sliding correlators, which has low current consumption, is used at a standby time.